In a typical known charge transfer process a photoconductive layer on a conductive substrate is situated in close proximity to a dielectric receiving layer, also present on a conducting substrate. When a sufficiently high voltage is applied between the two substrates, a dielectric breakdown occurs in the very small air gap between the two substrates, allowing charge transfer from the photoconductive layer to the dielectric receiving layer. Typically just prior to imaging, the system is biased with a voltage just below that required for the air-gap breakdown. Upon imagewise exposure, photocarriers, i.e., electrons and/or holes generated by the absorption of photons, created in the imaged areas of the photoconductive layer migrate in the applied field to increase the voltage across the air gap imagewise. Thus there is an imagewise transfer of charge across the gap from the photoconductive layer to the receiving layer. The electrostatic latent image on the receiving layer is then toned to develop the image.
To obtain good quality images it is desirable during the transfer step, to maintain a precise air gap between the photoconductive and receiving layers. Air gap separations of the order of a few microns are generally desirable. If the gap is too large, little or no charge will transfer; while if it is too small, there can be considerable transfer of charge in the background areas resulting in a mottled background. In addition, because the relationship between the voltage needed to cause dielectric breakdown in the air gap and the air gap spacing (the Paschen curve) is not constant, a uniform air gap spacing is desirable for high quality transfer images.
U.S. Pat. No. 2,825,814 teaches a method for maintaining spacing by placing between the surfaces of the photoconductive and receiving layers a small quantity of powdered resin or plastic which is obtained by grinding the material to a relatively uniform particle size. Disadvantages of this technique are: (1) the dusted particles tend to adhere to both surfaces after the charge transfer operation is complete and the surfaces are separated; (2) upon toning, the final image areas often contain blotches caused by the presence of the particles used to maintain the spacing; (3) the resin particles are not of uniform size and thus the spacing is not uniform; and (4) the particles used for spacing move slightly if utmost care is not taken when the two layers are separated after transfer of a latent or developed image. These disadvantages result in poor transferred images upon toning.
U.S. Pat. No. 3,519,819 discloses maintaining spacing by coating on a suitable substrate, e.g., paper, a thin layer of electrically insulating, solid, film forming polymeric binder containing particulate spacer particles randomly dispersed throughout the layer and embedded therein, e.g., substantially inert particles of various inorganic or organic materials. These particles are embedded in the polymer binder layer in such a manner that a portion of each protrudes above the surface of the layer. The amount by which these spacer particles protrude determines the air gap thickness. However, because the particle size distribution of the spacer particles is random and each particle is not deposited in the same orientation within the binder, the amount by which each particle protrudes above the substrate is not uniform. Particles deeply embedded in the binder would not be effective as spacers, while particles loosely embedded can become dislodged during use. Even when apparently uniformly sized spherical particles are used, the particles can become dislodged. If the particles are too closely spaced image clarity can be affected. Thus a uniform air gap cannot be achieved readily.